A new study published in Nature Astronomy reports the detection of plutonium-244 (Pu-244) in a deep-sea crust from the Pacific Ocean, providing evidence of a rare astrophysical event that occurred more than 100 million years ago.
The research, conducted by an international team from Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the Australian Nuclear Science and Technology Organisation (ANSTO), and the Australian National University (ANU), analyzed a ferromanganese crust sample for traces of radioactive isotopes.
"The ability to measure signatures from an event over 100 million years ago in a sample tens of millions of years old is a significant achievement."
Key Findings and Methodology
The study focused on a 1.9-kilogram ferromanganese crust sample recovered in 1976 from a depth of 4,830 meters in the Pacific Ocean. Researchers used multiple dating techniques and isotope detection methods.
- Plutonium-244 Detection: Plutonium-244, which has a half-life of 81 million years, was detected using accelerator mass spectrometry at ANSTO's VEGA facility. Only a few hundred atoms of the isotope were found per kilogram of crust.
- Curium-247 Absence: Curium-247 (Cm-247), with a half-life of 16 million years, was not detected in the sample.
- Iron-60 Signals: Two distinct peaks of iron-60 (Fe-60), a tracer for nearby supernovae, were identified in the crust. These peaks correspond to supernova events that occurred approximately 2 and 7 million years ago.
- Dating and Measurement: Beryllium-10 dating was performed at HZDR's DREAMS facility to establish an age model for the crust layers. Iron-60 measurements were conducted at the Heavy Ion Accelerator Facility in Canberra.
Interpretation of the Results
The distribution and presence of specific isotopes led the research team to conclude that the plutonium-244 originated from a source separate from the supernovae recorded by the iron-60 signals.
- Timeline: The presence of plutonium-244 combined with the absence of curium-247 indicates the cosmic event occurred more than 100 million years ago. The curium has fully decayed due to its shorter half-life, while enough plutonium remains to be detected. According to Dr. Michael Hotchkis of ANSTO, the absence of curium indicates the event was "a very long time ago," but the detectability of plutonium suggests it was "not more than about 1 billion years ago."
- Event Type: The most likely source is a rapid neutron capture process (r-process) event, such as a neutron star merger (kilonova) or a rare, high-energy supernova. The plutonium was found spread evenly across the crust layers, independent of the iron-60 signals, suggesting a distinct event.
- Ruling Out Alternatives: The findings reportedly rule out several theoretical models, including a recent proposal that the Solar System collided with a dense interstellar cloud.
"The absence of curium indicates the event was a very long time ago, but the detectability of plutonium suggests it was not more than about 1 billion years ago."
Implications and Future Research
The discovery provides constraints on the frequency and proximity of r-process events near the solar system. Such events produce heavy elements, including plutonium, through rapid neutron capture.
- The event was likely more powerful than a standard supernova. Researchers note that astrophysical events capable of producing these elements occur 1,000 to 10,000 times less frequently than standard supernovae.
- Dr. Dominik Koll (ANU and HZDR) noted that the ability to measure signatures from an event over 100 million years ago in a sample tens of millions of years old is a significant achievement. He stated that further modeling and experiments are needed to clarify details of the r-process.
The team plans to analyze lunar samples from NASA to study the last r-process event in more detail. The newly established HAMSTER facility in Dresden is expected to aid in detecting additional rare radionuclides.